Cleaning is essential as a preliminary to many surface finishing operations. Cleaning is normally required, for example, prior to corrosion preventive treatments and prior to the application of organic finishes and printing inks to metal surfaces. Cleaning is especially important in the case of metal surfaces to which organic materials have been applied as an aid to rolling or forming since these materials must be removed in order to obtain a surface which is receptive to printing inks and organic finishes.
Cleaners have been utilized in the manufacture of aluminum and tin plate, drawn and ironed cans. In the manufacture of such cans, circular blanks of aluminum or tin-plated steel are first cupped and then passed through several drawing dies to iron the cup in order to form a unitary sidewall and can bottom structure. These forming operations are assisted, and the dies and metal surface protected by the application of lubricants to the aluminum or tin surface prior to or during the forming operation. Since it is desired to have a clean surface prior to any processing steps such as conversion coating and sanitary lacquer deposition in order to assure adhesion of subsequently applied organic coatings, the cleaning step after forming is critical to a successful manufacturing process.
Most aluminum or tin plate can washing operations employ six sequential wash or rinse stages:
Stage 1: Prewash PA1 Stage 2: Acid or Alkaline Cleaner PA1 Stage 3: Rinse PA1 Stage 4: Condition or Treatment (conversion coating, mobility enhancing, etc.) PA1 Stage 5: Rinse PA1 Stage 6: Deionized Water Rinse PA1 (A) at least one inorganic base; PA1 (B) at least one cationic surfactant which is a quaternary ammonium compound; and PA1 (C) water.
During each stage, a bath containing the desired wash, cleaner or rinse composition is employed. This wash or rinse composition is preferably applied to the cans via spraying although other application techniques such as immersion can be used. Following stage 6, the cans are dried and then conveyed to a work station wherein they are further processed (e.g., printed, lacquered, painted, etc.).
Aluminum and tin plate cleaning or washer processes utilize and require the disposal of large quantities of water laden with chemical wastes. Because of environmental concerns, the water must be treated to reduce or remove chemicals from the waste stream, and new environmental regulations are requiring industries to spend a great deal of money on new waste treatment facilities.
During the cleaning process, organic soils such as forming, rolling and tramp oils, and inorganic soils such as metal fines, hard water salts, natural oxides and corrosion are removed from the metal surfaces. For example, if the metal is aluminum, the cleaning process removes oil, aluminum fines, aluminum oxides and water-soluble aluminum derivatives. When the metal is tin plate, the inorganic soils removed during the cleaning process include tin fines, tin oxide and water-soluble tin derivatives.
The inorganic soils which are present on the aluminum or tin cans are digested by the cleaner and remain in solution as a soluble metal salt. The oils which are utilized in preparing the aluminum or tin-plate coils and cans including forming oils and rolling oils which remain on the can surface are removed by alkaline or acid cleaners, and such organic soils are either insoluble or emulsified and dispersed throughout the cleaning solution. The organic soil must be removed from the surfaces of the cans because these soils are responsible for off flavors in the industry known as Labox. Additionally, incomplete removal of organic soils causes poor lacquer adhesion staining, spotting, or imperfections in subsequent coating and printing operations, and a reduction of the brightness of the aluminum or tin surface.
As the container cleaning cycle continues, the inorganic and organic soils accumulate in the cleaning and subsequent rinse solutions as contaminates. If uncontrolled, these contaminates can render the cleaning solutions and rinse solutions ineffective. The reduction of the contaminates in the cleaning and rinse solutions generally has been achieved through overflowing and/or discarding of the used solutions as fresh, unused solution is added. As noted, these methods of reducing contaminates result in the disposal of large quantities of water laden with chemical wastes. Moreover, this process requires that the cleaning solutions be replaced by fresh water and additional chemicals thereby increasing costs.
U.S. Pat. No. 4,028,205 (Dorsey) teaches using anionic or nonionic surfactants in alkaline cleaners for use on aluminum surfaces. The anionic and nonionic surfactants are disclosed as being useful for retarding the etch on the aluminum surface.
U.S. Pat. No. 4,094,701 (Fekete) describes a method for cleaning a tin surface. In particular, Fekete describes a cleaner which is an alkaline aqueous composition containing one or more surfactants and an organic tannin component to inhibit etching of the tin surfaces. The aqueous alkaline compositions are also reported to be useful for cleaning surfaces of other substrates such as steel and aluminum.
U.S. Pat. No. 4,477,290 (Carroll) describes alkaline cleaners for aluminum can bodies wherein the alkaline cleaner contains a metal chelating agent. The metal chelating agent promotes cleaning under soil loading conditions.
U.S. Pat. No. 4,528,039 (Rubin et al) describes alkaline cleaning compositions for aluminum surfaces which contain a mixture of alkali metal metasilicate and a compound such as sodium carbonate, potassium carbonate, potassium orthophosphate, etc. Surfactants may also be included, and these include nonionic, anionic, amphoteric or zwitterionic surfactants.
U.S. Pat. No. 4,540,444 (Kelly) describes the use of an alkaline cleaner containing a gluconate, an alkali metal phosphate and a surfactant to clean aluminum cans and to prevent off-flavors (Labox). Nonionic surfactants are disclosed.
U.S. Pat. No. 4,599,116 (King et al) describes aqueous alkaline cleaning compositions for aluminum containers which are effective in removing aluminum fines and organic soils from the aluminum container surfaces. The aqueous alkaline cleaning compositions contain an alkalinity agent, a complexing agent, one or more surfactants and, optionally, foam depressing agents. The patentees also mention the use of a pre-wash to remove a portion of the aluminum fines and soluble oil prior to the alkaline cleaning stage. The complexing agent is included in the alkaline bath in an amount effective to complex at least some of the metal ions in the operating bath which would otherwise tend to form bath-insoluble precipitates. Examples of the complexing agents include sugar acids and salts such as sodium gluconate and sodium citrate. The surfactants which are included in the alkaline cleaning composition are selected to remove the organic soils present on the substrate being cleaned and to prevent a build-up of such organic soils in the cleaning solution.
U.S. Pat. No. 4,762,638 (Dollman et al) describes alkaline cleaners for aluminum surfaces. The alkaline cleaning compositions comprise an ethylenediamine tetraacetic acid or nitrilo acetic acid alkali metal salt, an inorganic alkali metal phosphate, a surfactant and, optionally, an aluminum sequestrant, other inorganic salts and an alkali metal hydroxide if needed to adjust the pH of the composition to at least 11.0. The patentees indicate that the surfactant can be anionic, cationic or nonionic or combinations thereof (Col. 4, lines 15-17).